1. Field of the Invention
The field of the invention relates to high purity chemical delivery systems, in particular refillable high purity chemical delivery systems.
2. Description of Related Art
The chemicals used in the fabrication of integrated circuits must have a ultrahigh purity to allow satisfactory process yields. As integrated circuits have decreased in size, there has been a directly proportional increase in the need for maintaining the purity of source chemicals. This is because contaminants are more likely to deleteriously affect the electrical properties of integrated circuits as line spacing and interlayer dielectric thicknesses decrease.
One ultrahigh purity chemical used in the fabrication of integrated circuits is tetraethylorthosilicate (TEOS). The chemical formula for TEOS is (C.sub.2 H.sub.5 O).sub.4 Si. TEOS has been widely used in integrated circuit manufacturing operations such as chemical vapor deposition (CVD) to form silicon dioxide films. These conformal films are generated upon the molecular decomposition of TEOS at elevated temperatures and reduced pressures (LPCVD), or at lower temperatures in plasma enhanced and atmospheric pressure reactors (PECVD, APCVD). TEOS is typically used for undoped and doped interlayer dielectrics, intermetal dielectrics, sidewall spacers and trench filling applications.
Integrated circuit fabricators typically require TEOS with 99.999999+% (8-9's+%) purity with respect to trace metals. Overall, the TEOS must exhibit a 99.99+% purity. This high degree of purity is necessary to maintain satisfactory process yields. However, it also necessitates the use of special equipment to contain and deliver the high purity TEOS to the CVD reaction chamber.
Traditionally, high purity TEOS has been fed to the CVD reaction chamber from a small volume container called, an ampule. Historically, it was strongly believed ampules could not be metallic and that no metal should interface with the high purity TEOS or other source chemicals in the ampule. The use of metal ampules was spurned in the industry on the basis of the belief that high purity TEOS and other high purity source chemicals used in the semiconductor fabrication industry would pick up contamination from the metallic container in the form of dissolved metal ions. Thus, the industry used, almost exclusively, quartz ampules.
When these relatively small quartz ampules were emptied, they would simply be replaced with a full ampule. The ampules were not refilled in the fabrication area. The empty ampule was returned to the chemical manufacturer who would clean and refill the ampule.
Inconveniences resulting from the use of the quartz ampules are that they require frequent replacement due to their small size, which increases the potential for equipment damage. Furthermore, quartz ampules are subject to breakage, and have limited design versatility. Also, quartz has limited heat capacity making it difficult to control temperature of the ampule.
In an attempt to solve the problem associated with quartz ampules, at least one supplier of ultrahigh purity chemicals, Advanced Delivery & Chemical Systems, Inc., going against the general belief in the industry that high purity source chemicals should not be placed in contact with metal, developed a stainless steel ampule. This ampule was used to directly supply high purity TEOS and other high purity source chemicals to semiconductor fabrication equipment. As with the quartz ampules, when it was empty it was not refilled, but rather returned to the supplier for cleaning and refilling. There were still several problems with using the stainless steel ampule. Namely, because of the small size of the these ampules, they often required frequent replacement. Also, an optical sensor employing a quartz rod was used to detect when the high purity TEOS reached a low level inside the ampule. Unfortunately, optical sensors, which employ a light emitting diode and a photodetector in combination with a quartz rod, require a high degree of maintenance because they are subject to misalignment if jostled. Furthermore, the conditioning circuit of the sensor must be constantly tuned because the sensor is subject to calibration drift, which can cause false sensor output signals. These problems can result in allowing the ampule to run dry. Another problem with optical sensors is that they are prone to breakage in transport and cleaning, requiring frequent replacement. Despite these problems, optical sensors were used over more reliable metallic float sensor systems because of the fears of contaminating the high purity chemical with metal ions.
In an attempt to solve the problem of frequent replacement of stainless steel ampules, a larger five gallon stainless steel tank was developed to refill the smaller stainless steel ampule. This tank also used an optical level sensor to detect when the container had been depleted, despite all of the problems associated with optical level sensors. Like the ampule, this tank was not refilled, but was rather returned to the supplier for cleaning and refilling. Furthermore, in this configuration a second optical sensor, with all of the problems associated with such sensors, was required in the ampule to signal when the ampule was full during the refilling process. This in some cases required another opening in the ampule which is undesirable, because this introduces additional leak and contamination points. Due to the size of the five-gallon tank, it is subject to more physical jarring than the smaller ampules when transported and changed out with empty canisters, thus causing even more problems with the traditional optical sensors used to detect a low level of source chemical in the delivery system.
In an attempt to overcome the problems associated with the optical sensors, a metallic level sensor was employed to detect low levels of high purity in the five-gallon bulk container. The metallic level sensor generally consisted of a toroidal shaped float made of stainless steel held captive on a hollow shaft made of electropolished stainless steel. The float contained a fixed magnet. A digital reed relay was secured at a fixed position inside the shaft at an alarm trigger point. As the float travelled past the reed relay, the fixed magnet would change its state, thus causing a low level alarm condition to be signaled. A replacement tank would then be substituted. The digital magnetic reed relay used in the metallic float level sensor provided much more reliable detection of low source chemical levels in the remote tank, because the magnetic reed switch is a low maintenance mechanical switch and provides positive on/off switching. As before, the empty 5-gallon container was never refilled by the user. It was always returned to the chemical supplier for cleaning and filling.
A low level metallic float sensor has also been used more recently in the stainless steel ampule. Due to fears associated with contamination, however, they were not refilled by the user and were only used in stand alone systems. As with the five-gallon tank, when the metallic level sensor indicated the high purity TEOS or other high purity source chemical level was low, the ampule was simply replaced with a full ampule. In no instance was a metallic level sensor used to detect the level of high purity TEOS or other high purity source chemical in an ampule when the ampule was used in any refill type system. Ampules used in refill type systems have not used a float type sensor or any other sensor with movable parts.
The use of metallic level sensors has been spurned in ampules used in refill type systems because of the strong belief in the industry that sliding metal to metal contact will cause the shedding of metal ions and thus contamination of the high purity TEOS or other high purity source chemical employed in the delivery system. This belief exists despite the use of low level metal float sensors in stand alone stainless steel five-gallon tanks and in stainless steel ampules. This is because in the stand alone systems, the tank or ampule is exchanged with a replacement tank or ampule, respectively, following each use. Furthermore, following each use, the tank or ampule is cleaned before refilling for a subsequent use. Both the cleaning and refilling are accomplished at a remote location by the supplier of the source chemical. Therefore, the amount a metal float travels in a stand alone system is limited to one fill and drain cycle. On the other hand, in a refill system the ampule is periodically refilled from a remote bulk container after each time it is emptied. Further, in a refill system, the ampule is never completely drained of high purity TEOS or other high purity source chemical between each refilling. Thus, integrated circuit manufacturers and source chemical suppliers have been concerned that with repeated fillings of the same ampule, over a period of time the metal ion concentration in the ampule would increase to an unacceptable level. As a result of this concern, ampules that have been used in refill type systems have always been equipped with the optical sensors or with sensors with non-movable parts, despite the knowledge that metallic float level sensors were much more reliable in refill systems.
Because, as noted above, optical sensors require a high degree of maintenance and are subject to frequent failure, the reliability of the bulk chemical refill systems using optical sensors have been in question. When the optical sensor fails to detect a low or "empty" level, the ampule can be ran dry during the CVD process. As previously discussed, this would destroy the batch of wafers then in process at a cost of thousands of dollars. On the flip side, when the optical sensor fails to detect the high or "full" level during a refill cycle, the ampule can be overfilled potentially causing damage to costly equipment; wasting expensive high purity source chemical (high purity TEOS costs approximately $2,000/gal.); contaminating the fabrication area, which is typically a class 100 clean room environment; contaminating other equipment in the clean room; losing the wafers; and causing severe personal safety concerns. In the past, to avoid these problems semiconductor manufacturers have either not used refill systems, employed an operator to monitor the refilling process carefully, or taken the risk of an uncertain procedure outcome. At present, a need exists for a reliable bulk chemical refill system for applications where a high degree of chemical purity must be maintained, and a high level of error free refill confidence must exist.